1. Field of the Invention
The present invention relates to: a solid-state image capturing device (e.g., MOS image sensor) for transferring, to a voltage conversion section, signal charge obtained by a photoelectrical conversion at a photodiode that functions as a light receiving section for receiving light of a subject and sequentially reading a signal corresponding to a voltage obtained by a conversion at the voltage conversion section; and an electronic information device (e.g., digital camera, cell phone device equipped with camera and the like) using the solid-state image capturing device for an image capturing section thereof.
2. Description of the Related Art
Recently, a MOS (Metal Oxide Semiconductor) image sensor using a MOS has begun to be widely used as a conventional solid-state image capturing device, along with a CCD (Charge Coupled Device) image sensor. The reason for this is because it is possible, for example, to manufacture the MOS image sensor using a conventional IC (Integrated Circuit) manufacturing technique, and it is also possible to miniaturize the MOS image sensor and to improve the speed thereof by mounting a peripheral circuit for driving the MOS image sensor on the same chip. In addition, the MOS image sensor has an advantage compared to the CCD image sensor that it does not require a high drive voltage, and the structure thereof is simple.
Different from the CCD image sensor, each pixel section in the MOS image sensor requires: a photodiode that functions as a light receiving section for receiving light of a subject; a voltage conversion section for converting signal charge from the photodiode to a signal voltage; and a plurality of transistors that form a signal reading circuit, in order to read a signal from the photodiode. More specifically, for a plurality of transistors, for example, a commonly-used MOS image sensor requires: a charge transfer transistor for transferring signal charge from a photodiode to a voltage conversion section; a reset transistor for resetting the signal charge accumulated at the voltage conversion section prior to the signal charge transfer; an amplification transistor for amplifying and reading the signal charge accumulated at the voltage conversion section as a signal; and a selection transistor for selecting a pixel section to be read and outputting the signal amplified by the amplification transistor to a signal line. Accordingly, each pixel section requires: a photodiode; a voltage conversion section; and four transistors. Hence, this makes the reduction of a pixel section size difficult. As such, proposals have been recently made to suppress the characteristic deterioration that results from the reduction of a pixel section size by reducing the number of transistors for each pixel by employing a structure in which a voltage conversion section is shared by a plurality of pixel sections, a structure in which a circuit drive is performed without a selection transistor or the like.
For a device separation layer for separating transistors and photodiodes in each unit (e.g., in each pixel section), LOCOS (Local Oxidation of Silicon) is used. Further, with a recent advanced miniaturization, STI (Shallow Trench Isolation) is now commonly used for a device separation layer.
The commonly-used MOS image sensor described above forms an embedded photodiode structure by providing a surface diffusion layer at the top surface of a photodiode in order to suppress the flow of unwanted current (dark current) that is generated at the interface between a silicon substrate and a silicon oxide film into the photodiode. However, there also exists a numeral number of defects at the interface between a device separation layer and the silicon substrate, and thus there is much signal charge to be generated as noise.
In order to suppress the flow of charge that is generated at the interface between the device separation layer and the silicon substrate into the photodiode, Reference 1 proposes, for example, a structure of a pixel section in which a high-concentration semiconductor layer having an opposite polarity of a photodiode (or having an opposite conductive type of source and drain regions of a transistor) is formed to surround the side surfaces and the bottom surface of a device separation section that is formed by STI, in order to prevent the diffusion of unwanted electrons to the photodiode. This will be described with reference to Portion (a) of FIG. 9 and Portion (b) of FIG. 9.
Portion (a) of FIG. 9 is a top view showing an exemplary structure of a pixel section 100 in a conventional solid-state image capturing device disclosed in Reference 1. Portion (b) of FIG. 9 is a longitudinal cross-sectional view of a portion cut by line D-D′ in Portion (a) of FIG. 9.
As shown in Portion (a) of FIG. 9 and Portion (b) of FIG. 9, in the pixel section 100 in the conventional solid-state image capturing device, an n-type photodiode 102 having an n-type impurity implanted therein is formed at the top surface of a p-type semiconductor layer 101 and a p-type surface diffusion layer 103 is formed at the top surface of the photodiode 102 to form an embedded photodiode structure.
A transfer gate electrode 106 of a charge transfer transistor 110 is provided on the p-type semiconductor layer 101, which is between the photodiode 102 and a voltage conversion section 104, via a gate insulating film 105 made from a silicon oxide film.
A device separation insulating film 107 that is formed by STI is provided around the periphery of the pixel section 100 to separate adjacent photodiodes 102 from each other. On the D side of the portion cut by line D-D′, a p-type surface diffusion layer 108 is provided to surround the side surfaces and the bottom surface of the device separation insulating film 107. A p-type diffusion layer 109 is provided below the p-type surface diffusion layer 108 at a location deeper than the device separation insulating film 107. In this manner, by surrounding the side surfaces and the bottom surface of the device separation insulating film 107 with the p-type surface diffusion layer 108, leak current is prevented from flowing from the device separation insulating film 107 into the photodiode 102.
In addition, on the D′ side of the portion cut by line D-D′, the p-type diffusion layer 109 is provided at a location deeper than the device separation insulating film 107 to surround the side surfaces and the bottom surface of each of the device separation insulating film 107 and the voltage conversion section 104.
In addition, Reference 2 proposes, for example, an implantation separation structure in which a device separation section for separating adjacent photodiodes (or a device separation section for separating transistors) from each other is formed as an impurity-implanted diffusion layer, and an amount of surfaces of a device separation insulating film and a photodiode that face each other is reduced. Compared to the case where the device separation insulating film 107 that is formed by STI is provided around the periphery (around the four sides) of the photodiode 102 and the device separation insulating film 107 is surrounded by the semiconductor layer as shown in Reference 1 in Portion (a) of FIG. 9 and Portion (b) of FIG. 9, in the case of Reference 2, the device separation insulating film that is formed by STI is provided around the two sides of the photodiode on the charge transfer transistor side, the device separation insulating film is surrounded by a semiconductor layer, and the other two sides of the photodiode between adjacent photodiodes are only separated by employing an implantation separation structure. As a result, dark current can be reduced by a reduced amount of the device separation insulating film formed by STI. This will be described with reference to Portion (a) of FIG. 10 and Portion (b) of FIG. 10.
Portion (a) of FIG. 10 is a top view showing an exemplary structure of a pixel section 200 in a conventional solid-state image capturing device disclosed in Reference 2. Portion (b) of FIG. 10 is a longitudinal cross-sectional view of a portion cut by line E-E′ in Portion (a) of FIG. 10.
As shown in Portion (a) of FIG. 10 and Portion (b) of FIG. 10, in the pixel section 200 in the conventional solid-state image capturing device, an n-type photodiode 202 having an n-type impurity implanted therein is formed at the top surface of a p-type semiconductor layer 201 and a p-type surface diffusion layer 203 is formed at the top surface of the photodiode 202 to form an embedded photodiode structure, as in the case shown in FIG. 9.
A transfer gate electrode 206 of a charge transfer transistor 210 is provided on the p-type semiconductor layer 201, which is between the photodiode 202 and a voltage conversion section 204, via a gate insulating film 205 made from a silicon oxide film.
On the E′ side of the portion cut by line E-E′, a device separation insulating film 207 that is formed by STI is provided around the periphery of the pixel section 200 to separate adjacent photodiodes 202 from each other, and a p-type diffusion layer 209 is provided at a location deeper than the device separation insulating film 207 to surround the side surfaces and the bottom surface of each of the device separation insulating film 207 and the voltage conversion section 204.
On the E side of the portion cut by line E-E′, a p-type surface diffusion layer 208 having an impurity implanted and diffused therein is provided around the periphery of the pixel section 200 to separate adjacent photodiodes 202 from each other. The p-type diffusion layer 209 is provided at a location deeper than the p-type surface diffusion layer 208 to surround the side surfaces and the bottom surface of the p-type surface diffusion layer 208. In this manner, by forming the device separation section with the p-type surface diffusion layer 208, a stress upon a substrate by the device separation insulating film 207 that is formed by STI is reduced, and thus leak current can be suppressed.
Further, Reference 3 proposes, for example, a structure in which a thick oxide film is formed on a silicon substrate, and an impurity diffusion layer is provided below the oxide film for device separation to prevent the generation of dark current without employing LOCOS or STI for device separation. This will be described with reference to Portion (a) of FIG. 11 and Portion (b) of FIG. 11.
Portion (a) of FIG. 11 is a top view showing an exemplary structure of a pixel section 300 in a conventional solid-state image capturing device disclosed in Reference 3. Portion (b) of FIG. 11 is a longitudinal cross-sectional view of a portion cut by line F-F′ in Portion (a) of FIG. 11.
As shown in Portion (a) of FIG. 11 and Portion (b) of FIG. 11, in the pixel section 300 in the conventional solid-state image capturing device, an n-type photodiode 302 having an n-type impurity implanted therein is formed at the top surface of a p-type semiconductor layer 301 and a p-type surface diffusion layer 303 is formed at the top surface of the photodiode 302 to form an embedded photodiode structure, as in the cases shown in FIG. 9 and FIG. 10.
A transfer gate electrode 306 of a charge transfer transistor 310 is provided on the p-type semiconductor layer 301, which is between the photodiode 302 and a voltage conversion section 304, via a gate insulating film 305 made from a silicon oxide film. A thick insulating film 307 is provided around the periphery of the pixel section 300 on a silicon substrate to separate adjacent photodiodes 302 from each other. A p-type surface diffusion layer 308 having an impurity implanted therein is provided below the insulating film 307. In this manner, by forming the device separation section with the p-type surface diffusion layer 308 and the thick insulating film 307, the insulating layer (i.e., the thick insulating film 307) is not embedded at a deep location in the substrate, and the occurrence of crystal defect or damage on the semiconductor substrate around the device separation section, or the generation of interface state on the semiconductor substrate around the device separation section is suppressed. As such, noise resulting therefrom can be reduced.
In addition, since the four sides of the photodiode 302 are surrounded by the thick insulating film 307 and the transfer gate electrode 306 is provided on the insulating film 307, the p-type semiconductor layer 301 and the transfer gate electrode 306 are separated by the thickness of the insulating film 307. Thus, charge is less likely to flow into an adjacent photodiode 302 of a neighboring pixel section via the transfer gate electrode 306 (cross talk). In this case, the transfer gate electrode 306 is provided on the top surface of the p-type semiconductor layer 301 via a thin gate insulating film 305.
Reference 1: Japanese Laid-Open Publication No. 2004-253729
Reference 2: Japanese Laid-Open Publication No. 2002-270808
Reference 3: Japanese Laid-Open Publication No. 2005-347325